Peripheral blood mononuclear cells (PBM) obtained from patients with active untreated systemic lupus erythematosus (SLE) were evaluated both for the number of cells spontaneously secreting immunoglobulin (Ig) as well as for their capacity to generate immunoglobulin-secreting cells (ISC) in vitro in response to pokeweed mitogen (PWM). ISC were enumerated by a reverse haemolytic plaque assay designed to quantify the number of cells secreting IgG, IgM and IgA. PBM obtained from eight patients with active untreated SLE contained markedly increased numbers of ISC compared to age-, sex-, and race-matched normal control PBM. SLE PBM contained a mean of 13,805 +/- 3266 ISC per 10(6) cells, of which 74% secreted IgG, 10% IgM and 22% IgA, while normal PBM contained a mean of 779 +/- 143 ISC per 10(6) cells, with 57% secreting IgG, 25% IgM and 33% IgA. PBM obtained from SLE patients were also examined for their ability to generate ISC in vitro in response to PWM. SLE PBM were markedly deficient in their capacity to respond to PWM with the differentiation of ISC. This diminished responsiveness could not be ascribed to serum factors, the presence of increased numbers of cells with suppressive capacity or the absence of potentially responsive B cells. Rather, a deficiency of helper T cell activity appeared to be responsible. This was indicated by the observation that PWM responsiveness could be restored to SLE PBM by co-culturing them with purified mitomycin C-treated normal T cells.